The present invention relates to a vehicle having a transmission and an engine whose tendency to knock differs from one fuel to another different in octane number.
Tendency of engine to knock differs from one fuel to another in octane number. Thus, in a vehicle having a transmission drivingly connected to such an engine, it is the conventional practice to set the transmission at a single state corresponding to fuel supplied to the engine.
In a known vehicle having a continuously variable transmission disclosed in laid-open Japanese patent application 54-157930, a ratio between the rotational speed of a driving pulley and the rotational speed of a follower pulley is adjusted to a desired value by controlling distribution of fluid pressure between a fluid pressure chamber of the driving pulley and a fluid pressure chamber of the following pulley and thus controlling the distance of each of V-shaped pulley grooves receiving a V-belt in response to various signals, including an engine speed indicative signal, representing an operating state of the vehicle. In this known transmission, a shifting speed at which the transmission effects shifting from one ratio to another is constant under the same condition and is dependent upon rate of change in fluid pressure supplied to each of fluid pressure chambers of the driving and follower pulleys. This shifting speed should be determined taking into account the tendency of the engine to knock and thus should vary from one case where regular octane fuel (octane number ranging from 75-85) is used to another case where high octane fuel (octane number above 85) is used because the tendency of engine to knock differs from the regular octane fuel to the high octane fuel. Commonly, a shifting schedule of the continuously variable transmission is set such that the engine operates at a speed falling into a low speed range. In setting the shifting speed, if it is set high as quick as possible to such a degree that the tendency of the engine to knock remain at a low allowable level so as to meet the case where it operates on fuel having a high octane number, the tendency of the engine to knock will exceed the allowable level when it is supplied with fuel having a low or regular octane number if the transmission effects shifting toward a small ratio to suppress the engine speed because the shifting toward the small ratio is effected at the same and fast speed, too. If, the shifting speed is set slow enough to lower the tendency of the engine to knock so as to meet the case where the engine operates on the regular octane fuel, the fuel is wasted during shifting toward a small ratio in the case where the engine is supplied with the high octane fuel because it takes a time to complete the shifting to the small ratio so that engine speed remains high during this period of shifting. Therefore, it is hardly possible to set the shifting speed at a value which meets the case where the regular octane fuel is used as well as the case where the high octane fuel is used.
This problem also arises in a vehicle having a discretely variable automatic transmission. If a shifting point at which the automatic transmission shifts is set at a low vehicle speed so as to effect shifting at a low engine speed range for the purpose of improving fuel consumption, the engine tends to knock in the case where regular octane fuel is used. If the shifting point is set at a high vehicle speed, the tendency of the engine to knock is suppressed in the case where the regular octane fuel is used, but the fuel is consumed at an unnecessarily high rate.
With the same engine, a high power output is produced if the engine operates on high octane fuel as compared to an engine power output produced if the engine operates or regular on low octane fuel. The spark timing of the engine should be adjusted to different values corresponding to the use of high octane fuel and that of regular octane fuel, respectively. Laid open Japanese patent application 58-143169 discloses an engine wherein the spark timing is automatically adjusted to different values corresponding to the use of the high octane fuel and that of the regular octane fuel by a control unit employing a knock sensor.
Line pressure of the automatic transmission is variable in a pattern corresponding to the pattern of variation of the power output of the engine. If the line pressure is adjusted to meet the pattern of the power output of the engine when it operates on high octane fuel, the magnitude of the line pressure becomes excessively large when the engine is to operate on regular octane fuel, resulting in an increase in shocks during shifting. If the line pressure is adjusted to meet the pattern of the power output of the engine when it operates on the regular octane fuel, the magnitude of the line pressure becomes too low when the engine is to operate on the high octane fuel, failing to provide a sufficiently high capacity of each of the frictional units, resulting in a slip and baking of the unit.
The present invention, therefore, aims at solving the above mentioned problems arising when an engine operates on different kinds of fuel in octane number selectively.